1. Field of the Invention
The present invention relates to an axial flow fan for axially moving air, and more particularly to an axial flow fan capable of achieving reduction of noise while having a high blowing efficiency compared to the power consumption of a motor adapted to drive the axial flow fan.
2. Description of the Related Art
Generally, axial flow fans are configured to axially move air while rotating in accordance with the driving operation of a drive motor. Such an axial flow fan may be effectively used in motor vehicles to promote heat discharge of a heat exchanger such as a radiator or condenser.
Referring to FIGS. 7 and 8, an axial flow fan for a motor vehicle is illustrated. As shown in FIGS. 7 and 8, this axial flow fan includes a hub 1 coupled to the drive shaft of a drive motor, and a plurality of blades 2 extending radially around the hub 1. The axial flow fan for motor vehicles may also selectively include an annular fan band 3 surrounding the blades 2 while connecting tips of the blades 2 together. The annular fan band 3 serves to guide a radial flow of air in an axial direction, thereby obtaining an enhanced axial blowing efficiency. The annular fan band 3 also supports the blades 2 so as to avoid a deformation of those blades 2.
In such an axial flow fan configuration, each blade 2, which is configured to directly guide air in an axial direction, has a streamlined cross-sectional structure. Each blade 2 sucks air from the upstream side of the axial flow fan and discharges the sucked air toward the downstream side of the axial flow fan by virtue of an increase in pressure occurring at the pressure surface of the blade when it rotates, thereby blowing air. The structure of the blade has a great influence on the blowing efficiency and noise of the axial flow fan.
In particular, axial flow fans for motor vehicles should meet various performance requirements for different applications thereof, respectively.
For example, in the case of an axial flow fan used in a motor vehicle, it is used to cool a radiator adapted to cool the engine of the motor vehicle, and a condenser adapted to improve the performance of an air conditioner. In this case, the axial flow fan has to generate a sufficient amount of air flow to cool the heat exchangers while withstanding loads applied to the heat exchangers, that is, static pressure drops. Also, the axial flow fan should exhibit a high blowing efficiency compared to the power consumption of its motor, taking into consideration problems related to vehicle battery capacity due to a recent tendency to mount an increased number of electronic appliances to vehicles. Moreover, the axial flow fan should generate a reduced level of noise to meet the standing rule for the reduction of noise. In addition, the axial flow fan should not be damaged even when it rotates at high speed.
In designing an axial flow fan having a configuration meeting the above mentioned performance requirements, the shape, width, and mounting angle of blades are handled as important design factors because those blades have a great influence on blowing efficiency and generation of noise.
As shown in FIGS. 7 and 9, conventional axial flow fans have blades 2 each swept in a rotating direction. The sweeping degree of the blade 2 in a rotating direction, that is, the sweep angle σr, is increased as the blade 2 extends radially toward its tip 2b (for example, the blade portion connected to a fan band 3 where the fan band 3 is installed). Thus, conventional axial flow fans selectively have either forward blades having a structure in which each blade is swept in a rotating direction, or backward blades having a structure in which each blade is swept in a direction opposite to the rotating direction.
For example, an axial flow fan having forward blades will be described with reference to FIG. 9. As shown in FIG. 9, this axial flow fan includes blades 2 each having a structure in which a leading edge LE, that is, the end of the blade 2 in a rotating direction, a trailing edge TE, that is, the end of the blade 2 in a direction opposite to the rotating direction, and a median line ML between the leading and trailing edges LE and TE are swept in the direction opposite to the rotating direction of the fan at a root 2a of the blade 2 secured to a hub 1 while being swept in the rotating direction as they extend from the root 2a to a tip 2b of the blade 2. That is, the sweep angle σr, which is defined as an angle between a radius line R extending from the center of the hub 1 through an arbitrary point and a tangent line TL at the optional point and represents a slope of the blade 2 in the rotating direction, is backward (−) at the root 2a of the blade 2, while being inflected at a particular point IP (hereinafter, referred to as an “inflection point”) spaced away from the blade root 2a, to be forward (+) at the tip 2b of the blade 2. That is, the blade 2 of the axial flow fan has a backward sweep angle σr (σr<0) at a root region thereof (that is, a region radially inward of the inflection point IP of the blade 2) while having a forward sweep angle σr (σr>0) at a tip region thereof (that is, a region positioned radially outwardly of the inflection point IP of the blade 2).
In such an axial flow fan, a flow concentration region C is formed near the inflection point IP of each blade where the sweep angle σr of the blade is changed from backward to forward. By virtue of the flow concentration region C, the axial flow fan has been appreciated as generating less noise, as compared to other axial flow fans.
U.S. Pat. No. 4,569,631 discloses an axial flow fan including blades having a sweep angle of at least 30°. Also, U.S. Pat. No. 4,684,324 discloses an axial flow fan including blades having a structure wherein the inflection point of each blade is defined within the range of 0.25 to 0.5 of the non-dimensional radius of the blade.
Although the conventional axial flow fans have the above mentioned forward or backward-swept blade structures, they cannot provide a satisfactory noise reduction effect. In order to achieve a quiet running of motor vehicles, therefore, development of an axial flow fan capable of achieving a satisfactory noise reduction has been strongly required.
To meet such a requirement, U.S. Pat. No. 5,906,179 has proposed an axial flow fan wherein each blade has a chord length, that is, the distance between the leading and trailing edges of the blade, which varies depending on a variation in the length of the blade, such that the chord length has a minimum value at a predetermined location. Also, U.S. Pat. Nos. 5,603,607 and 4,089,618 have proposed an axial flow fan having a blade trailing edge of a sawtooth shape.
Although the conventional axial flow fans respectively having the above mentioned blade structures achieve a limited reduction of noise, they cannot provide a satisfactory noise reduction effect. For this reason, where such axial flow fans are applied to a motor vehicle, they degrade a desired quiet running of the motor vehicle.